Beams of high-energy ions are commonly used in the field of medical radiation therapy. Use of proton beams in particular has become accepted for treatment of various types of cancer. The energy of the ion beam is an important parameter in radiation treatment, since it determines the depth of beam penetration. The depth of beam penetration is used by treatment planning systems to generate treatment plans for individual patients.
A calibration system is used to determine the depth of treatment of a beam as a function of the energy setting of the beam generator. Current calibration systems measure the depth of penetration of the beam in a water tank to simulate the beam penetration in a patient. An example of such a system is illustrated in FIG. 1A, which includes a beam generator and associated electromagnets 100, a water tank 110, an ion detection chamber 120, and a linear actuator 130. In operation, beam generator 100 generates a charged particle beam 140 (e.g., a charged particle pencil beam), which is directed to enter water tank 110. The beam 140 enters the tank 140 through an entry window 115 where it is detected by ion detection chamber 120, which is located at a variable distance or depth from the window 115. The ion detection chamber 120 measures the degree of ionization as a function of water depth, which is controlled by the linear actuator 130. The full curve of such a measurement as a function of water depth is referred to as the Bragg Curve. For a monochromatic proton beam, the Bragg Curve in pure water is well known, and is shown below for two representative initial proton energies (200 MeV and 250 MeV) in FIG. 1B.
Although water tank calibration is widely accepted, it can be prone to errors in operation and analysis and is not convenient to apply on a regular basis because of its size and weight. Alternative methods have been investigated, including the use of diode arrays to eliminate mechanical scanning. This technique suffers from low special resolution.
It would be desirable to have improved methods and systems for verifying the calibration of charged particle beams.